almost all of the system is fed by gravity, without the use of energy-consuming pumps. valves open to regulate the flow into the 85-mile-long delaware aqueduct -- the longest tunnel in the world. at hillview reservoir... the water is partitioned into another giant tunnel system. where it travels deep below manhattan. the pressure built up by gravity from the mountains pushes the water upwards toward the surface through vertical shafts. these shafts feed the water mains of each neighborhood, which branch into smaller pipes below the streets... feeding into buildings and houses, into the plumbing, and finally, after its long journey, to our faucets. providing water to homes and industry is a monumental task, requiring immense infrastructure. but once the water is delivered and used, it must also be taken away. man: it's important that the waste generated by any society not be left around. cholera, and other diseases and problems, have been spread, because people wound up living in filth. even the ancients understood that you couldn't have the sewage where you lived. and t